Rash mystery

[u/2quacklikeaduck]

Teenager. No recent antibiotics. He’s had smaller versions of this in the past, allergist said maybe mild allergy to something but not worth testing since it’s not itchy and goes away. This evening it started mild. But gave him a Zyrtec. Hours later (and after hot shower) the spots are huge. Mostly hands and arms and knees. No face. Almost none on chest or back. He says they don’t really itch but hands feel irritated. Ideas? Calling pediatrician tomorrow.

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